In a conventional method for absolute value summation and subtraction, subtraction of two operands X and Y is carried out, wherein one of the two operands X and Y, for instance, the operand Y is converted to a two's complement which is then added to the operand X to provide a difference D. In this case, where the operand X is smaller than the operand Y (X&lt;Y), the difference D becomes a two's complement for an absolute value .vertline.X-Y.vertline.. Therefore, a sequence for calculating the two's complement is necessary to indicate the difference D in the expression of an absolute value. In the calculation of a two's complement of the difference D, summation between an inverted difference D, in which each bit of the difference D is inverted, and a number having weight at the least significant bit (LSB) is carried out. In this case, the summation is often realized with a high speed by use of carry look ahead. This is to be explained in more detail later.
However, the conventional method for absolute value summation and subtraction has a disadvantage in that subtraction takes a long time to deteriorate a high speed processing thereof, because a sequence for calculating an absolute value of a subtracting result, which is essentially a summation sequence, is inevitable for the subtraction dependent on a comparison in value of two operands.